This invention relates in general to vehicle brake drums and in particular to an apparatus and method for measuring the circularity and eccentricity of a brake drum while the brake drum is mounted upon a vehicle.
Most vehicles are equipped with a brake system for retarding or stopping movement of the vehicle in a controlled manner. A typical brake system for an automobile or a light truck includes a disc brake assembly for each of the front wheels and either a disc brake assembly or drum brake assembly for each of the rear wheels. The brake assemblies are actuated by hydraulic or pneumatic pressure which is generated when the vehicle operator depresses a brake pedal.
A typical drum brake assembly includes a rigid backing plate which is secured to a stationary portion of the vehicle, such as the end of a tubular axle housing. The backing plate carries a brake shoe assembly which includes a pair of arcuate-shaped brake shoes and associated return springs. The drum brake assembly also includes an actuator which is operatively connected to the brake shoes. In a brake system actuated by hydraulic pressure, the actuator is usually embodied as a brake cylinder. In a brake system actuated by pneumatic pressure, the actuator is usually embodied as a cam actuator. In both types of brake systems, an access slot is usually formed through the backing plate. The access slot allows insertion of brake adjustment tools into the brake assembly. Such a brake adjustment slot is typically included on all light trucks and automobiles weighing less than 10,000 pounds (4,536 KG).
A typical drum brake assembly further includes an annular brake drum which completely surrounds the brake shoe assembly. Often, a portion of the brake drum extends over the backing plate so as to prevent water and dirt from entering within the brake drum assembly. The brake drum is provided with an inner braking surface which is disposed about the brake shoe assembly. The brake drum is bolted between a rotatable wheel hub and a vehicle wheel. Thus, when the vehicle wheel turns, the brake drum rotates with it about the stationary brake shoe assembly. Usually, the wheel hub is provided at the end of an axle shaft rotatably supported within the axle housing. Ideally, the inner braking surface of the brake drum is precisely cylindrical in shape, and the axis of rotation of the brake drum is precisely concentric with the axis of rotation of the axle shaft.
During operation of the brake system, hydraulic or pneumatic pressure is generated when the operator depresses the brake pedal. As a result, the actuator urges the non-rotating brake shoes outwardly toward the inner braking surface of the rotating brake drum. As the brake shoes contact the inner braking surface, friction generates a braking torque which slows the rotation of the vehicle wheel and, if maintained, will completely stop such rotation.
In order to achieve a smooth braking action, two factors are important. The first factor is that the inner braking surface of the brake drum be precisely circular. Brake drum circularity is indicative of the amount of variation in the inner braking surface from a perfectly round cylinder. One manner of measuring brake drum circularity is to measure the difference between the maximum and minimum diameters of the inner braking surface. Another manner of measuring brake drum circularity is to measure the maximum change in braking surface radius over a sector having a predetermined size, such as thirty degrees. This latter measurement is commonly referred to as a "rate of change", although neither time nor mathematical differentiation are involved in the calculation thereof. In either event, improper brake drum circularity is usually the result of a defect in the structure of the brake drum.
The second factor for achieving smooth braking action is that the inner braking surface of the brake drum be precisely concentric with respect to the axis of rotation of the axle shaft upon which it is mounted. Brake drum eccentricity is indicative of the amount of variation between the axis of rotation of the brake drum with respect to the axis of rotation of the axle shaft. Brake drum eccentricity usually results from the stack-up tolerances of the various wheel mounting components and the mounting holes of the brake drum. Additionally, brake drum eccentricity can result from distortion of the brake drum, such as can be caused by uneven torquing of the wheel nuts. Improper brake drum circularity is, therefore, usually the result of defects in the manufacture or assembly of the wheel mounting components.
When a brake drum is mounted upon a vehicle, the combined effect of brake drum circularity and brake drum eccentricity is often referred to as the "installed runout" of the brake drum. If the installed runout is excessive, the braking action which occurs when the brake shoes engage the inner braking surface of the brake drum will be rough. Rough braking action is undesirable for a number of reasons, including safety and comfort. Thus, it is desirable to measure brake drum circularity and brake drum eccentricity to prevent this from occurring.
To measure brake drum circularity, it is known in the art to remove the brake drum from the vehicle and measure the inside diameter thereof at various locations. If the maximum inside diameter exceeds a predetermined value, the drum is considered to be defective. It also is known in the art to measure the inside diameter of the brake drum while the brake drum is mounted upon the vehicle. An apparatus and method for doing so is described in U.S. Pat. No. 4,782,595 to Diewert.
To measure installed runout, it is known in the art to remove the brake drum from the vehicle and use relatively sophisticated test equipment to measure both the brake drum circularity and brake drum eccentricity components. By using such test equipment, the individual values of brake drum circularity and brake drum eccentricity can be determined. It is also known to measure the installed runout of the brake drum while it is mounted upon a vehicle. However, known test equipment for measuring the installed runout of the brake drum while it is mounted on the vehicle cannot determine the individual values of brake drum circularity and brake drum eccentricity. Inasmuch as the removal of the brake drum from the vehicle for testing is relatively time consuming and expensive, it would be desirable to provide an improved test fixture which is capable of determining the individual values of brake drum circularity and brake drum eccentricity while the brake drum is mounted on the vehicle.